For several years already, it has been known to fit transparent substrates, especially glazing panels, with conducting tracks which can act as heating elements or as antenna or alarm elements.
These tracks are generally obtained by the screen-printing method using a paste containing metallic silver particles. It is known from EP-A-0 712 814 that the paste has a high silver content, that is to say from 60 to 90% by weight of the solid material. Moreover, EP-A-0 079 854 describes a paste, capable of being deposited by screen printing on glass, which comprises from 45 to 90% by weight of metallic silver particles with a size less than 1 μm.
The electrically conducting tracks may also be obtained by methods other than screen printing, for example by extruding a conducting thermosetting paste directly on the glass in order to form narrow wires (see DE-A-1 796 310).
The electrically conducting tracks which are obtained after baking (which baking is generally carried out at the same time as treating the glazing panels for forming and/or toughening purposes) have sufficient mechanical strength. As a result, the additional galvanizing step, which is tricky to implement because of the pollution risks associated therewith, is avoided.
Glazing panels comprising electrically conducting tracks are very widespread in the automobile field. Most often, these tracks are employed as heating tracks, especially on rear windows, but they can also be placed on the glazing panel to provide it with an alarm and/or antenna functions. The aforementioned documents give no indication with regard to the width of the electrically conducting tracks thus produced. In practice, electrically conducting tracks are formed in industry by conventional screen printing and, after baking, they have a width of between 0.4 and 1.2 mm and a thickness which varies according to the nominal heating power and the ohmic resistance per unit area in question.
Because of the beneficial functions provided by these tracks, their number on one and the same glazing panel has tended to increase over the years, which could pose problems of overall size and of visibility. Thus, when the tracks are located in the field of vision of the glazing panel, they are clearly visible from the inside, which may bother the driver, and secondarily from the outside, which is detrimental to the esthetic appearance of the vehicle.
Moreover, it is already known to use screen-printing stencils to form varied patterns on the glass (see DE-A 32 31 382 and DE-A-35 06 891). Thus it is also possible to apply the paste in a thicker and/or wider layer at certain locations of the glass in a single step (without multiple printing), for example in order to form busbars for the electric current of the heated glazing panels. In this way, it is possible for the temperature over the entire glazed surface to be adjusted as well as possible, this temperature not having to exceed 50° C. in the region of the busbars under standard ambient temperature conditions for a heating power going up to 450 watts. The examples appearing in the aforementioned patent applications are produced with a d.c. voltage commonly employed for automobiles, of about 11 to 14 volts.
Heated glazing panels and glazing panels with antenna, the conducting tracks of which consist of fine tungsten wires with a diameter of a few micrometers, are also known. These wires are only present on laminated glazing panels and they are embedded within the adhesive forming the intermediate sheet since otherwise it is not possible for them to be fastened safely directly onto the glass. Since they are finer, these wires are consequently less visible than the conducting tracks obtained by screen printing.
There is a requirement by automobile manufacturers, especially for top-of-the-range vehicles, to have available glazing panels made of toughened or laminated safety glass provided with conducting tracks which are barely visible to the naked eye.